Four-stroke cycle engines, which operate on 720 degrees of crankshaft revolution and employ hydraulic valve actuation systems, are known. One such hydraulic valve actuation system is disclosed in Patent Literature 1 below, i.e. JP-A2005-201259 (=EP 1 555 398 B1 entitled “Internal combustion engine having a single camshaft which controls the exhaust valves mechanically, and the intake valves through an electronically controlled hydraulic device”). In this known system, the intake valves of the engine have the respective pumping pistons actuated by the respective cams of the single camshaft. Each of the pumping pistons transmits a thrust to a stem of the corresponding one of the intake valves to cause the opening of the intake valve against the action of a return spring by means of hydraulic fluid present in a pressure chamber and by means of a valve actuating piston.